Polypeptides having physiological activity are useful as therapeutic agents for specified diseases, but there are many cases in which sufficient pharmacological effects cannot be expected because of their poor stability when they are administered into blood. For example, there is a case in which the physiological activity is lost due to degradation by a hydrolase or the like existing in blood. Further, in the case of exogenous physiologically active polypeptides, their physiological activities are sometimes effective in treating diseases. However, since structures of such exogenous polypeptides, polypeptides produced by gene recombination, and the like are different from those of endogenous polypeptides, it is known that they induce an immune response when administered into blood and sometimes cause serious side effects such as anaphylactic shock or the like. In addition, among the physiologically active polypeptides, there are many polypeptides in which physical properties such as poor solubility or the like become problems when they are used as therapeutic agents.
As one of the methods for solving these problems in using a physiologically active polypeptide as a therapeutic agent, a method in which an inactive polymer chain such as polyethylene glycol is chemically bound to the polypeptide is firstly known.
For example, polyethylene glycol modification of a granulocyte colony-stimulating factor (G-CSF) is known [Journal of Biochemistry, 115, 814-819 (1994)]. Additionally, examples of the polyethylene glycol modification of asparaginase, glutaminase, adenosine deaminase, uricase or the like have been reported [Pharmaceutical Biotechnology, Volume 3, Stability of Protein Pharmaceuticals, Part B, In Vivo Pathways of Degradation and Strategies for Protein Stabilization, edited by Tim J. Ahern and Mark C. Manning, (USA), Plenum Publishing Co., November 1992, pp. 235-263]. As the effects obtained by modifying physiologically active polypeptides with a polyalkylene glycol, increase in heat stability [Biophysics, 38, 208 (1998)], solubilization in organic solvents [Biochemical and Biophysical Research Communications, 122, 845 (1984)] and the like are also known, in addition to the improvement of sustainability in blood, reduction of antigenicity and immunogenicity, improvement of stability in blood and the like.
However, when a physiologically active polypeptide is modified with a polyalkylene glycol, it is difficult in many cases to improve blood stability without decreasing physiological activity of the polypeptide. In general, it is known that blood sustainability of the polypeptide is improved as the molecular weight of a polyalkylene glycol becomes large or the degree of modification becomes large [Journal of Biochemistry, 263, 15064 (1988)], but the physiological activity of the polypeptide is decreased in some cases when the degree of modification is increased. It is considered that the modification of specific amino acid residues such as an amino group, a mercapto group, or the like which are necessary for the physiological activity, by a chemically modifying agent, the inhibition of the interaction at the physiologically active site by the polyalkylene glycol bound to the physiologically active polypeptide, or the like, are responsible therefor. Interleukin-15 is known as an example in which its physiological activity is reduced depending on the degree of modification [Journal of Biochemistry, 272, 2312 (1997)]. Thus, chemical modification has many advantages, but since the physiological activity of the polypeptide is generally decreased by the chemical modification, a chemically modifying agent which does not decrease the physiological activity of the polypeptide when its modification is carried out is desired.
On the other hand, it is sometimes difficult to prepare an aqueous solution of a low molecular compound having a concentration thereof capable of showing the physiological activity because many low molecular compounds have low water-solubility. In addition, even when an aqueous solution may once be prepared, there is a case in which the precipitate is formed during its preservation, thus causing substantial reduction of the concentration in a solution. For example, compound such as many anticancer agents, antibiotics, antiviral agents and the like are exemplified as such compounds. Accordingly, when a physiochemical test is carried out or a therapeutic agent is administered to the living body, it is probable that not only the intended effective concentration cannot be obtained, but also there is a case in which the pharmacological effect of interest cannot be achieved. Thus, attempts have been made to use low molecular compounds which hardly dissolve in water by solubilizing them through mixing with or chemically binding to a polymer or a surfactant. For example, a case is known in which a copolymer of polyethylene oxide and aspartic acid was used for the solubilization of amphotericin B [Journal of Controlled Release, 53, 131-136 (1998)]. Also, a case is known in which cyclodextrin was used for the solubilization of amphotericin B [Biopolymers, 28, 1585-1596 (1989)]. In addition, bile acid micelle and the like are also used for the solubilization of low molecular compounds such as cholesterol or the like. Polyethylene glycol and the like are also used for improving solubility of an anticancer agent [Cancer Research, 50, 1693-1700 (1900)]. As described above, methods and reagents for the solubilization of low molecular compounds without decreasing their physiological activities are desired. Also, a method in which a solubilizing agent is added as described above requires a necessity to take into consideration a problem of reproducibility and an influence of the solubilizing agent itself. Accordingly, reagents and methods capable of improving solubility or stability by chemically modifying low molecular compounds themselves are desired.
On the other hand, dendrimers having glycerol in their structures are known [Macromolecules, 34, 7648-7655 (2001); Polymer Preprints, 42, 155-156 (2001); Polymer Preprints, 42, 157-158 (2001); Journal of American Chemical Society, 123, 2905-2906 (2001); Journal of American Chemical Society, 122, 10335-10344 (2000); Chinese Journal of Chemistry, 16, 28-33 (1998)].